A class of polymers known as polyimides has become known for its combination of good heat stability and high upper use temperatures, as measured by glass transition temperature. A particularly useful type of such polyimides is known as polyimidesiloxanes.
Because of their combination of properties, polyimidesiloxanes have been used in electronic applications, particularly in micro-electronic components in the computer industry.
Because the previously known polyimidesiloxanes are insoluble or difficultly soluble in solvents, when used in the micro-electronics industry, there is a great need for polyimidesiloxanes having improved solubility characteristics, as well as a better balance of heat resistance and upper use temperature.
The chemistry for making polyimides has been well-known since about 1960. A structurally simple polyimide can be prepared by reacting a diamine with a dianhydride. ##STR1##
The first step, or the polycondensation reaction, generates polyamide acids which are hydrolytically unstable even at room temperature. The second step, or the imidization reaction, produces the stable polyimides desired for various applications.
Polyimidesiloxanes can be prepared by reactions employing siloxane diamines or siloxane dianhydrides with organic comonomers. Polyimidesiloxanes can also be prepared from siloxane diamines and siloxane dianhydrides without an organic comonomer.
Only a few polyimidesiloxanes are soluble, even in high boiling and relatively toxic solvents, such as 1-methyl-2-pyrrolidinone (NMP), despite the fact that most of their polyamide acids are soluble. The usage of polyamide acids in coating applications has many drawbacks. First, a subsequent imidization reaction on substrates produces water. Therefore, it can only be used in very thin film coatings and where void-free property is not critical to performance. Second, the removal of high boiling, polar solvents, such as NMP, requires temperatures as high as 350.degree. C. for about 30 minutes even for films of a micron thickness. This drying process is not only energy intensive, but also unacceptable to some heat sensitive electronic devices or substrates. In addition, the polyamide acids solution has to be stored at refrigeration temperature (&lt;4.degree. C.) and it still has a very short shelf life (about 3 months). Finally, only the fully imidized polyimidesiloxanes are thermally stable for melt processing such as extrusion and injection molding. A soluble polyimidesiloxane can be fully imidized at temperatures of about 160.degree. to 170.degree. C. in a solvent, whereas imidization for insoluble polyimidesiloxanes in the solid state may require temperatures 50.degree. C. above their glass transition temperatures which can be as high as 200.degree. to 250.degree. C. Shaping not fully imidized polyimidesiloxanes by the melt processing method produces voids in the products and often is not desirable.
Therefore, one object of the present invention is to prepare fully imidized polyimidesiloxanes.
Polyimides derived from reactions of 4,4'-oxydiphthalic anhydride (4,4'-ODPA) and organic diamines have been studied by Frank W. Harris and Lyn H. Lanier (in F. W. Harris & R. Seymour Edt., "Structure--Property Relationship in Polymers", Academic Press, 1977, pp. 182-198) and also by T. L. St. Clair et al, (Ibide. pp. 199-213). These polyimides, as well as other BTDA containing polyimides based on benzophenone tetracarboxylic dianhydride (BTDA), are very insoluble. For instance, out of 12 polyimides prepared in T. L. St. Clair's report, there are only two polyimides that are soluble in NMP and none are soluble in diglyme or methyl ethyl ketone (MEK). Diglyme is diethylene glycol dimethy ether, also known as 2-methoxyethyl ether. Polyimides prepared from oxydiphthalic anhydride and complex diamines are disclosed in U.S. Pat. Nos. 3,705,869 and 3,705,870. U.S. Pat. No. 4,048,142 discloses the use of 3,3'-oxydiphthalic anhydride in making polyimides.
The first polyimidesiloxane was prepared by reacting pyromellitic dianhydride (PMDA) with 1,3-bis-(aminopropyl)-1,1,3,3-tetramethyl disiloxane in 1966 (see V. H. Kuckertz, Macromol. Chem. 98, 1966, pp. 101-108). This polyimidesiloxane is a crystalline material and cannot be cast into flexible films from solvent. Polyimidesiloxanes derived from reactions of benzophenone tetracarboxylic dianhydride (BTDA) and .alpha.,w-diamino organo-polysiloxanes were disclosed by General Electric in 1967 in U.S. Pat. No. 3,325,450. Polyimidesiloxanes containing an .alpha.,w-diamino organo-polysiloxane and a diether dianhydride (DEDA) have also been disclosed in U.S. Pat. No. 3,847,867.
All these BTDA and DEDA containing polyimidesiloxanes are amorphous materials. They have a glass transition temperature of no more than 100.degree. C. and, therefore, have very limited upper use temperatures, despite the excellent thermal stability of these polymers up to about 200.degree. C.
Polyimidesiloxanes containing both organic and siloxane monomers have been reported for PMDA containing copolymers (see Japan Kokai Tokkyo Koho Nos. 83/7473 and 83/13631); for BTDA containing copolymers (U.S. Pat. Nos. 3,553,282 and 4,404,350) and for diether dianhydride containing copolymers (U.S. Pat. No. 3,847,867). These PMDA containing polyimidesiloxanes are not soluble in any solvent. The BTDA containing polyimidesiloxanes are only soluble in high boiling or toxic solvents such as N-methyl pyrrolidone (NMP), phenol or cresol, and the like. The diether dianhydride containing polyimidesiloxane, in addition, are also soluble in chlorinated solvents such as dichlorobenzene and dichloromethane. Since these phenol and chlorinated compounds are both corrosive and highly toxic, the polyimidesiloxanes have limited application in coating applications, especially in heat sensitive electronic devices. This is also due to the fact that a NMP soluble polyimidesiloxane normally has to be heated to 350.degree. C. for at least half an hour to remove all the residual solvent in a film having a micron-thickness film.
Some diether dianhydride containing polyimidesiloxanes are soluble in diglyme (T.sub.b =162.degree. C.) and may be sparingly soluble in tetrahydrofuran (T.sub.b =60.degree. C.); but none of these polyimidesiloxanes are soluble in solvents such as methyl ethyl ketone (T.sub.b =80.degree. C.) which is one of the most used solvents in the coating industries. However, all these polyimidesiloxanes have relative low glass transition temperatures (below about 125.degree. C. to 150.degree. C.) and limited thermal stability (350.degree. C./0.5 hour with retention of film flexibility and integrity). Thermally stable polyimidesiloxanes which are soluble in non-toxic and low boiling solvents such as diglyme or methyl ethyl ketone, are not readily available.
U.S. Pat. No. 4,395,527 to Berger discloses a large number of various components as useful in manufacturing polyimidesiloxanes. While the use of oxydiphthalic anhydride is disclosed in the patent there is no recognition that this compound would provide particularly useful properties. Moreover, the tolylene diamine used in the present invention is not even disclosed in the cited patent.
U.S. Pat. No. 4,586,997 teaches the utility of making polyimidesiloxanes based on diether dianhydrides, diamines and .alpha.,w-diaminosiloxanes. Cross-linked polymers are also disclosed. There is no recognition that the use of oxydiphthalic anhydride, which is a monoether dianhydride, as the sole essential organic dianhydride in the polymer produces a polymer with exceptional properties.
Another object of the invention is to provide fully imidized polyimidesiloxanes which are soluble not only in high boiling solvents, such as NMP, but also in low boiling, low toxic, less polar solvents such as diglyme, tetrahydrofuran (THF) or methyl ethyl ketone (MEK). A further object of the invention is to provide polyimidesiloxanes that have a good balance of heat resistance and high upper use temperatures, as measured by glass transition temperatures.
Another object of this invention is to produce curable and cross-linked polyimidesiloxanes.